The present technology for producing mechanically strippable thin copper foil on an aluminum carrier involves the following combination of steps:
Clean to remove oil from aluminum carrier PA1 Water rinse PA1 Etch to provide a uniform matte surface PA1 Water rinse PA1 Treat with acid to neutralize etchant and to desmut PA1 Water rinse PA1 Apply copper electroplate from standard pyrophosphate copper bath to build up a thickness of up to about 50 microinches. PA1 Water rinse PA1 Build up thickness of copper with any conventional copper electroplating bath. PA1 20-40 g/l copper as copper sulfate pentahydrate PA1 50-60 g/l sulfuric acid PA1 10-22 nitrate as ammonium nitrate PA1 310 g/l zinc sulfate PA1 27 g/l aluminum sulfate PA1 22 g/l sodium sulfate
While this procedure can and does produce very satisfactory mechanically strippable ultra-thin foils of copper on an aluminum carrier, it does have its limitations. For example, the initial stages of copper deposition produce a sparse distribution of relatively large copper nuclei. This in turn limits the rate at which a relatively non-porous copper layer providing full coverage is developed.
Attempts to increase the density of copper nuclei by adjusting the parameters of the existing process have not been fruitful.
The advantage of an increase in density of smaller copper nuclei is tha it results in an initial copper plate which is less porous than one developed from a relatively sparse distribution of nuclei. Further, a higher density of copper nuclei will result in more complete coverage with the application of a thinner layer of copper than attainable with present technologies. This has certain economic advantages.
Further, the ability to obtain good coverage and low porosity with less copper reduces the required contact time with the copper bath, which can result in an increased line speed in a continuous plating operation.
Some improvement in the direction of decreasing contact time has been accomplished by increasing the current density of the pyrophosphate copper plate cell. However, there is a limit to which current density can be increased before burning of the copper plate takes place.
The prior art has recognized that some improvement in adhesion and corrosion resistance in copper plating aluminum can be achieved by first treating the aluminum with an alkaline, aqueous, alkali metal zincate treating bath. Such zincate baths, as described in U.S. Pat. Nos. 2,676,916 and 2,650,886, contain an alkali metal hydroxide, zinc oxide, a water soluble metal salt, such as a salt of iron, cobalt or nickel, and a complexing agent for the metal ions introduced as the water soluble metal salt, such as Rochelle salt, a tannate, or a sugar. These prior art patents also teach that the thickness of the zincate coating may be controlled by the addition of minor amounts of a nitrate or nitrite.
Since these zincate treating baths are designed to improve the adhesion between the subsequently applied copper plate and an aluminum carrier, they would not be useful in the preparation of ultra-thin copper foils which are mechanically strippable from aluminum carriers.
It is also known in the art to anodize aluminum in the presence of ferric ions, and to anodize iron plated aluminum, prior to copper plating the aluminum. This procedure, according to Akiyama et al, "Electroplating on Anodized Aluminum" Plating Magazine, pp. 594-598, June, 1971, produces copper plates having "good" adhesion to aluminum.